Electrical resistance grid



Patented Jan. 18, 19 27.

UNITED STATES,

CHARLES T. EVANS, OF TITUSVILLE, PENNSYLVANIA.

ELECTRICAL RESISTANCE GRID.

No Drawing.

Thisinvention relates to an improved cast electrical resistancegrid and has for its object the provision of a cast grid which shall not have more expansion and contract1on 5 than would cast iron while having the strength and durability which are characteristic of steel.

A further object of the inventionis to provide a cast. electrical resistance grid which shall give a more nearly uniform resistance throughout the range of customary operating temperatures.

7 In carrying out my invention I utilize a ferrous alloy electrical resistance which when cast in grids has the above desired qualities and in addition thereto is nonwarping and does not'oxidize sufliciently to interfere with its use as an electric resistance.

I willnow give a formula for a ferrous alloy electrical resistance embodying the desired product in one of its preferred forms, the figures representing percentages by weight.

Per cent. Carbon 0.40 Chromium 7.50 Nickel 19.50

. Manganese .75 Silicon 1.25

The principal remaining ingredient is iron.

The above may be termed an ideal anal .sis which may not always be obtained n practice. In regular manufacturing operations this electrical resistance usually contains some phosphorus, and some sulphur. Theseare impurities which should be avoid- 40 ed as much as possible. Also some copper maybe found in the alloy, this usually being brought in with the iron. A small amount of copper, for example, about 0.35%, is not objectionable, particularly if the manganese content is kept as high as 1% or is increased somewhat above this amount.

important feature of my invention consists in preparing a starting mixture of such ingredients in such proportions as will,

when melted, give the new electricalresistance alloy hereinbefore described and adding-to this mixture, prior to or at the time of melting it, a small amount of zirconium, usually in the form of form-zirconium or silico-zirconium. By this addition the desired product is obtained with greater cer- Application filed August 14, 1922. Serial No. 581,813.

tainty and with less care in operation. By the use of a restricted amount of the zirconium in the mix no zirconium will be found on analysis of the resulting alloy, or, at most, only such a small amount that it cannot be detected by the ordinary methods of analysis employed in metallurgical works.

As one example of a.mix which may be employed, I submit the following:

, 1 Parts Iron 650 Nickel shot, 205 Ferro-chromium 108 Manganese metal 8 Ferro-silicon 14 Silico-zirconium 15.

The mix when melted in a crucible and cast in the usual way gave a product of WhlCh the following is the analysis:

Per cent. Carbon O. 374 Nickel 19. 88 Chromium 7. 52 Silicon 1. 22 Manganese 0. 685

The principal remaining ingredient was iron.

In making the mix it is advantageous to use an iron which has a low carbon content and which is freed from impurities as much as poss1ble, particularly'from sulphur and phosphorus. In other words, pig iron, for

added to the mix 1n about the) proportions I stated for the zirconium.

The mix may, of course, be melted in an electric furnace, but the crucible method has been entirely satisfactory. iThemolten material may be cast into the desired form of an electric resistance. directly from the furnace,or it may be run into an ingot mold and later remelted.

By the present invention an electrical resistance grid is rovided'having the desir able properties 0 a goodsteel and yet at the i same time not having a greater expansion and contraction than the former cast iron grids. A further and very important advantage obtained is that the grid of the present invention provides a more nearly uniform resistance throughout the range of ordinary operating temperatures and is capable of withstanding extremely high temperatures without warping or sagging and, in addition, is practically non-corrosive. By non-corrosive I mean that the resistance grid or element is subject only to a slight surface oxidation which does not penetrate rapidly to the interior of the resistance as with cast iron grids.

From several tests which have been made of grids made in accordance with my invention as compared with a well known cast iron grid, it has been found that while the cast iron grid had a resistivity of about 489 ohms per mil per foot, the new grid gave a resistivity of 518 ohms per mil per foot, these results being obtained at about 225 C.

Furthermore, the resistance of the new grids does not vary so much with tempera ture changes as does the old cast iron grid. In fact the cast iron grid, which had about the same resistance as my new grid at room temperature showed an increase of resistance 26% greater than that of my new grid when both were raised to 1038 C. I

The percentage compositions of grids hereinbefore given represent what I believe to be the best embodiment. of the invention. However, the invention is not to be considered as limited to those proportions, for good results may be obtained where the proportions are considerably varied. "For example, chromium may vary from 3.50 to 7.75; nickel from 10 to 20; carbon from 0.20 to 1.00; manganese from 0.25 to 2.50; silicon from 0.15 to 2.50, the remainder being principally iron. lVhen any copper is used it should be between 0.05 and 0.35. The figures given indicate percentages by weight.

\Vhat is claimed is:

1. A cast electrical resistance grid comprising the following ingrcdients in substantially the following percentages chromium 3.50 to 7.75; nickel 10 to 20; carbon 0.20 to 1.00; manganese 0.25 to 2.50; silicon 0.15 to 2.50; the remainder being principally iron.

2. A cast electrical resistance grid comprising the following ingredients in substantially the following percentages chromium 3.50 to 7.75; nickel 10 to 20; carbon 0.20 to 1.00; manganese 0.25 to 2.50; silicon 0.15 to 2.50; copper 0.05 to 0.35, the remainder being principally iron.

3. The process of making a ferrous alloy electrical resistance which process comprises making a mix capable of giving, when melted and cast, a ferrous alloy having the composition set forth in claim 1 and which comprises supplying to the mix' zirconium, in such proportion that it will not be found in the ferrous alloy on analysis of the same.

In testimony whereof, I have hereunto set my hand.

CHARLES '1. Evans. 

